Conventionally, an apparatus such as a laser microscope, an optical pickup apparatus, a laser processing machine, has been known which irradiates light beam to an object to thereby detect information on the shape, etc., of the object, or to produce some change in the object. In such an apparatus, it is desirable, in order to achieve high resolution, to obtain a beam spot radiated from a light source and focused on the object with size that is as small as possible. In general, the minimum diameter of the beam spot is defined by diffraction limit, and this minimum diameter is proportional to wavelength of light. Therefore, the shorter the wavelength of light radiated from the light source, the smaller is the diameter of beam spot.
However, a light source that emits light of short wavelength, i.e., a laser emitting violet light or ultra violet light is generally more expensive than a light source that emits light of longer wavelength, for example green light or red light. Also, in general, in the wavelength range from violet to ultra violet, light transmittance of optical material decreases as wavelength becomes shorter. Therefore, an optical material that exhibits high light transmittance for violet or ultra violet light is very limited.
Thus, radial polarization is now attracting much attention. Radial polarization is a polarized light beam in which polarization plane of linearly polarized light is distributed radially with the optical axis as a center. It is reported that, by focusing a light beam of radial polarization with a condenser to a focus, the focused light beam in the focus plane has Z-polarization (i.e., the direction of electric field is same as the direction of propagation of light), so that a light beam can be focused in smaller spot diameter than the spot diameter defined by diffraction limit of X- or Y-polarization). It is also reported that as the range of small beam diameter along the optical axis becomes longer, the larger a depth of focus can be achieved. (See, for example, Chin-Cherng Sun, Chin-Ku Liu, “Ultrasmall focusing spot with a long depth of focus based on polarization and phase modulation”, OPTICS LETTERS, Optical Society of America, 2003, vol. 28, No. 2, p. 99-101; and HAIFENG WANG, et. al., “Creation of a needle of longitudinally polarized light in vacuum using binary optics”, Nature photonics, 2008, vol. 2, p. 501-505).